Load diversion method and apparatus for head protective devices

ABSTRACT

A load-diverting helmet contains an interface layer disposed between inner and outer helmet layers. The inner helmet layer is attached to a helmet retention system that can be used to hold the inner helmet layer to a wearer&#39;s head. When a tangential impact force is applied to the outer helmet layer, the interface layer allows the outer helmet layer to displace with respect to the inner helmet layer, thereby absorbing and/or diverting forces that would be transferred in a conventional helmet design to the wearer&#39;s head and neck as rotational acceleration.

[0001] This application claims priority to U.S. Provisional PatentApplication No. 60/416,312, filed by Steven M. Madey on Oct. 4, 2002,and titled “Load Diversion Method and Apparatus for Head ProtectiveDevices”, the entirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to helmets and other headprotective devices, and more particularly to methods and apparatus forlessening the transfer of oblique impact forces to an individual wearingsuch a device.

BACKGROUND OF THE INVENTION

[0003] Traumatic brain injury (TBI) is the leading cause of death andlong-term disability in the U.S.A. among people below 45 years of age.In the U.S. alone, each year over two million people sustain traumaticbrain injury, with a financial impact of about $4 billion annually. Toalleviate the impact of TBI, a variety of task-specific helmets havebeen introduced. To date the use of helmets is no longer confined tohigh-risk occupational scenarios and motorcyclists, but appreciateswide-spread acceptance and even legal prescription for commonrecreational and sports activities, such as bicycling. Given the largenumber of helmets in use, even a small improvement in the protectiveeffect of helmets will evoke a considerable benefit to the health statusof the general population.

[0004] Helmets are designed to protect the brain and skull during animpact. Conventional helmets perform this function by distributing andabsorbing a portion of an impact's kinetic energy by deforming(elastically or inelastically) a compliant layer. Typically, apermanently attached outer shell distributes the impact load, and aninterior-padding layer absorbs impact energy. For an activity such asbicycling, the capabilities of the shell and padding layer represent acompromise between the need to maximize energy absorption and minimizeobject penetration, practical and aesthetic limitations on weight andthickness, and other factors such as style, aerodynamics, head cooling,etc.

[0005]FIG. 1 shows a traditional recreational sports hard shell helmet20 in longitudinal cross-section. A padding layer 22 surrounds and isgenerally shaped to fit (usually with insert foam padding to adjust fora comfortable fit) the protected portion of the wearer's head. A hardouter shell 24 of glass-reinforced plastic, polycarbonatethermoplastics, or the like, is adhered to padding layer 22. A helmetretention system (e.g., straps 26) allows the user to attach the helmetsecurely to the head, usually with a chinstrap and increasingly with asuspended rear strap or member that aligns the helmet properly on thewearer's head. The helmet retention system usually attaches directly toouter shell 24.

[0006]FIG. 2 shows, in cross-section, a micro-shell helmet 27, which haslargely replaced the traditional hard shell helmet for most bicycleapplications (primarily due to reduced weight). Helmet 27 uses a liner22 manufactured from expanded polystyrene beads, designed to absorbkinetic energy upon impact. A tape strip (not shown) running along thelower edge of a plastic microshell 28 secures the plastic microshell tothe exterior of liner 22. A helmet retention system 26 generallyattaches to and/or loops through holes in liner 22. Upon substantialimpact, the plastic microshell may tear apart or split, and theunderlying liner may crack or shatter as it absorbs energy.Consequently, microshell helmet manufacturers typically recommendreplacement after a single crash, even if helmet integrity appearsuncompromised.

[0007] Particularly with microshell helmet designs, large cross-sectionintegrated vents have been incorporated into the helmet as a sellingfeature. FIG. 3 illustrates a vented microshell helmet 30 inlongitudinal cross-section. Although similar in construction to helmet27, microshell 28 and liner 32 contain substantial shaped voids or ports(e.g., 34) that ostensibly function as vents to allow some airflowthrough the helmet. The remaining polystyrene struts 32 may be quitethick, in order to restore at least part of the structural integritylost due to voids 34.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention may be best understood by reading the disclosurewith reference to the drawing, wherein:

[0009]FIGS. 1, 2, and 3 show longitudinal cross-sections of three priorart recreational helmet designs;

[0010]FIGS. 4A and 4B contain cross-sections of a helmet according to anembodiment of the invention, shown respectively in longitudinal andtransverse cross section;

[0011]FIG. 5 shows construction and functional detail of the embodimentof FIGS. 4A and 4B;

[0012]FIGS. 6 and 7 show construction and functional detail of anotherembodiment employing a different interface layer;

[0013]FIGS. 8A and 8B illustrate yet another embodiment of theinvention, which incorporates air vents and uses an interface layerdisposed near the outer helmet shell;

[0014]FIGS. 9A and 9B depict an interface layer comprising a lamellarstructure, respectively under no-load and tangential force conditions;

[0015]FIGS. 10A and 10B depict an interface layer comprising adjacentrigid shells, respectively under no-load and tangential forceconditions;

[0016]FIGS. 11A, 11B, and 11C depict an interface layer employing thinconnecting members that pass through the interface layer, respectivelyunder no-load, small tangential force, and large tangential forceconditions;

[0017]FIG. 12 illustrates one possible displacement responsecharacteristic for a non-linear interface layer according to anembodiment of the invention;

[0018]FIG. 13 shows the construction of an embodiment wherein connectingmembers are disposed around the periphery of air vents in a helmet; and

[0019]FIG. 14 demonstrates test results for a standard helmet and anembodiment of the invention when subjected to an oblique impact with aconcrete surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The increased use of bicycle helmets over the past decade doesnot seem to have reduced the incidence rate of traumatic brain injuryper bicyclist, but has coincided with a 51% increase in the rate of headinjuries per active cyclist. This phenomenon may only insufficiently beexplained by a more aggressive riding attitude among bicyclists, basedon a misleading sense of security provided by helmets. It is nowrecognized herein that this seeming incongruity may be accounted for, atleast in part, by three specific deficiencies in the design of bicyclehelmets.

[0021] First, prior bicycle helmets are primarily designed to distributeand absorb impact loads by means of a padding layer underlying agenerally dome-shaped semi-rigid shell. This design is effective indistributing a focused impact over a larger area of the cranium toreduce the risk of skull fractures. Given geometric constraints of thepadding layer, however, its ability to absorb a significant amount ofenergy is limited. Subsequently, the remaining energy will betransformed into acceleration of the head, where the amount and durationof head acceleration directly correlates to the extent of traumaticbrain injury. Therefore, the design of contemporary bicycle helmets mayeffectively reduce the incident of skull fractures, but falls short inprotecting its user from the disabling consequences of traumatic braininjury.

[0022] Second, bicycle helmets are primarily designed to absorb impactloads by means of energy-absorbing and load-distributing layers. Thisdesign greatly increases the distance of the impact site (i.e., thesemi-rigid outer shell) from the apparent rotation axis of the headaround the neck, therefore providing an effective lever arm to transformthe tangential component of an oblique impact to the helmet into anangular acceleration of the head. Since the brain is most susceptible toangular acceleration of the head, current helmet designs do not protectthe head from closed-head traumatic brain injury due to angularacceleration, but may instead contribute to it.

[0023] Third, bicycle helmets are primarily designed to absorb impactloads by means of a padding layer underlying a generally dome-shapedsemi-rigid shell. Traditional helmets (for motorcycling, skateboarding,etc., see FIG. 1) employ a rigid, dome-shaped outer shell that tends tohold its shape and slide during impact. Modern bicycle helmets (seeFIGS. 2 and 3) have a smooth, semi-rigid, non-continuous surface made ofa considerably thin plastic layer. This thin shell primarily improvesappearance and aerodynamics but is essentially unsuitable to bear ordivert typical impact loads present during head impacts. As a method forenergy absorption, the helmet surface (in combination with an underlyingfoam core) will deform to congruency with the surface geometry of theimpacting object. Instead of diverting an impacting object and itsassociated kinetic energy, this congruency can lead to a prolongedimpact duration, and the corresponding form lock between the impactingobject and the helmet will cause an effective transfer of energy to thehead instead of an effective energy diversion.

[0024] In summary, the inefficacy of bicycle helmets to diverttangential impact energy may dramatically limit their ability todecrease the amount and magnitude of head acceleration. The use ofcontemporary helmets may therefore be ineffective to reduce theincidence of closed head traumatic brain injury, and may in factincrease the stresses to the cervical spine. In contrast, the presentinvention provides a means of diverting and/or absorbing tangentialimpact energy before that energy is translated to the wearer's head andneck.

[0025] A first embodiment of the present invention is illustrated ashelmet 40 of FIGS. 4A and 4B, in longitudinal and transversecross-section. Helmet 40 comprises an inner helmet layer (e.g., anenergy-absorbing layer) 41, a helmet retention system (e.g., straps 42and foam padding inserts, not shown) to affix inner helmet layer 41 to awearer's head, an outer helmet layer (the combination ofenergy-absorbing layer 43 and microshell 44 in this embodiment), and aninterface layer 45 disposed between the inner and outer helmet layers.

[0026] The inside surface of energy-absorbing layer 43 forms a cavitywith a spherical curvature. Likewise, the exterior of energy-absorbinglayer 41 has a spherical curvature with a slightly smaller radius thanthat of layer 43. During assembly, interface layer 45 is interposedbetween layers 41 and 43 and secured (e.g., by adhesive bonding) to theouter surface of layer 41 and the cavity surface of layer 43.

[0027] Interface layer 45 can provide, in essence, a spherical bearingof low/controlled friction to allow relative displacement between layers41 and 43 under oblique impact conditions. Accordingly, assembled helmet40 allows an inner helmet layer to remain affixed to a wearer's head,while allowing potentially large rotational displacement of outer layers43 and 44 with respect to the inner layer (and the wearer's head) inresponse to an applied force.

[0028]FIG. 5 illustrates further construction and operational detailsfor this first embodiment. In this embodiment, interface layer 45consists of a distensible flexible envelope filled to a desiredthickness with a viscous medium 52. Viscous medium 52 can be, forexample, a gel (e.g., silicone), a liquid (e.g., aqueous solutions, anoil, or other lubricant), and/or a filler comprising solid sphericalparticles. Segment 50 of the envelope is secured to the inner surface ofouter helmet layer 43; segment 54 of the envelope is secured to theouter surface of inner helmet layer 41. Along the bottom edge of theinterface, a “slack” section of the envelope can be left unattached, toreadily permit some limited rotation, with any remaining rotationrelying, for example, on distention of the envelope and/or partialseparation of the envelope from the attached helmet layers. The slacksection can also distend to displace portions of the viscous medium andabsorb compression loads placed on the envelope.

[0029]FIG. 5 depicts the application of an oblique force vector F to theouter helmet 44. Force vector F can be decomposed into two componentvectors, one (F_(norm)) normal to the helmet surface at the point ofimpact, and the other (F_(tang)) acting along the helmet surface. Themagnitude of F_(tang) will depend on the coefficient of friction betweenlayer 44 and the striking object. It is noted, however, that largeF_(norm) values will generally cause substantial deformation of layers43, 44 when such use a traditional microshell construction, therebyincreasing friction and F_(tang).

[0030] In response to force vector F_(tang), envelope 45 allows arotational displacement R of the outer helmet layers with respect to theinner helmet layers, absorbing and/or deflecting at least a portion of arotational component that would otherwise be communicated to thewearer's head. At the same time, envelope 45 may absorb a portion offorce vector F_(norm) as viscous medium 52 pressurizes, forcing envelope45 to distend along its periphery and allow some of viscous medium 52 tobe displaced from between layers 41 and 43. Geometric and constitutiveproperties of the envelope and viscous medium, such as medium thicknessand viscosity, the elastic modulus and area of unattached envelope atthe helmet periphery, and the force required to separate the envelopefrom other helmet layers, can be adapted to meet specificshock-absorbing and shear-deflecting/absorbing properties for specifichelmet applications.

[0031] An additional advantage that can be achieved in at least someembodiments is improved absorption of force vector F_(norm) due to thedisplacement of the impact site during impact. Materials such as foamwill completely compress, or “bottom out”, near the impact site for somevalue of F_(norm). To reduce this phenomenon in a prior art helmet,typically one increases the foam thickness. In an embodiment with adisplaceable outer layer, however, the area of the inner layer directlyunder the impact site will generally be changing over the duration ofthe impact. This movement increases the area over which the inner foamlayer is being compressed, thus potentially delaying and/or preventingbottoming out.

[0032]FIG. 6 illustrates a second interface layer constructiontechnique. In FIG. 6, interface layer 45 consists of a hyper-elastic gelthat can be bonded to the inner surface of layer 43 and the outersurface of layer 41 to ensure helmet integrity during normal use. Thegel allows rotational displacement—and potentially separation—of theouter helmet assembly with respect to the inner helmet assembly when atangential impact force is applied to the outer helmet. Further,depending on gel properties, layer 45 may act to absorb a portion ofF_(norm) and/or further distribute that force before it is applied toinner helmet layer 41. Geometric and constitutive properties ofinterface layer 45, such as its thickness, viscosity, and/or elasticmodulus can be adapted to meet specific shock-absorbing andshear-deflecting/absorbing properties for specific helmet applications.

[0033]FIG. 7 illustrates the same helmet cross-section shown in FIG. 6,but with a different impact force vector F typical of a cyclist goingover her handlebars and hitting the pavement headfirst. In FIG. 7, then,the resultant F_(tang) vector causes the outer helmet assembly to rotateforward to partially shield the wearer's face. Thus although notnecessary in every embodiment, the helmet geometry can be designed toextend a portion of the outer helmet layers 43, 44 down past theoriginal edge of the helmet due to displacement upon impact, potentiallyproviding increased facial protection in a crash. Such a feature is notnecessarily limited to forward rotation—with an appropriate design and alarge displacement, the outer helmet layer can generally be designed toact as an extended shield to protect areas of the head that wouldotherwise not be covered by a protective shell.

[0034] In the preceding embodiments, the interface layer was interposedbetween two, e.g., expanded polystyrene layers. FIGS. 8A and 8Billustrate another helmet embodiment 80, respectively in transverse andlongitudinal cross section, wherein an interface layer 84 is interposedbetween a rigid outer shell 85 and an energy-absorbing layer 81, 82.Preferably, rigid outer shell 85 is constructed of a material such asglass-reinforced or carbon fiber-reinforced plastics/resin systems,polycarbonate, titanium, or perhaps high-density polyethylene. Althoughsuch an embodiment can potentially use a microshell, a microshell maytend to crack and tear upon impact, exposing the interface layer andallowing the impact surface to ablate the underlying interface layer.Depending on interface layer construction, this characteristic may ormay not be tolerable—if not, a hard shell that resists deformation andtearing can be used.

[0035]FIGS. 8A and 8B show other features that may be desirable in aparticular application. For instance, helmet 80 incorporates air vents(e.g., 86), with the interface layer attached to support pillars 81, 82of the helmet. Helmet 80 also illustrates that the inner and outerhelmet layers need not have coextensive head coverage (see hidden line83 in FIG. 8B, showing the lower edge 83 of the interface layer 84. Inpractice, a design may limit the extent of the interface to helmet areasthat are most likely to strike the ground so as to cause rotationalacceleration, e.g., the frontal helmet quadrants, provided thatsufficient clearance is allowed for outer helmet assembly displacementupon impact.

[0036] Finally, helmet 80 shows a tail appendage 87 attached to theoutside of shell 85. Tail appendage 87 can be used to impart aerodynamicor aesthetic qualities to the helmet without impeding the displacementfunction of interface layer 84. Other constructs can have, for instance,part of shell 85 covering part of interface layer 84 and part of shell85 covering part of tail appendage 87, with tail appendage 87 attacheddirectly to a part of interface layer 84.

[0037] In addition to the filled envelope and hyper-elastic gelembodiments already described, other interface layer constructions arepossible. FIG. 9A shows a section of a helmet with an outer helmet layer90, an inner helmet layer 92, and an interface layer 94 disposed betweenlayers 90 and 92. Interface layer 94 comprises a lamellar structure ofhyper-elastic columns 96. Columns 96 buckle under an impact force toabsorb impact energy. And as shown in FIG. 9B, application of a forceF_(tang) to one of the helmet layers results in a relative displacementd as columns 96 bend and stretch in response to the tangential force,thereby deflecting and partially absorbing the tangential force.

[0038] The interface layer can also comprise multiple solid shelllayers, as shown in FIG. 10A. Although such layers can be buried betweenenergy-absorbing layers within the helmet, FIG. 10A shows an interfacelayer comprised of two solid shell layers 106 and 108 near the helmetexterior. Solid shell layer 106 attached to an outer helmet layer 100.Solid shell layer 108 attaches to an inner helmet layer 102. The solidshell layers can be designed to slide readily across each other inresponse to a force F_(tang) applied to the outer helmet layer (see FIG.10B), thereby deflecting the force instead of transmitting it to theinner helmet layer. Solid shell layers 106 and 108 can be constructed,for example, of materials that exhibit a low coefficient of friction,such as polyethylene or polytetrafluoroethylene (PTFE), and can furtheremploy an intermediate lubricant to further reduce friction betweenlayers 106 and 108.

[0039] Optionally, solid shell 106 can function alone as both outershell 100 and as part of the displaceable interface of the helmet insome embodiments.

[0040] Shells 106 and 108 can be held together initially in a fixedposition, e.g., by peripheral tape or another connecting member (to beexplained below) designed to shear upon impact, thereafter allowing free(or freer) motion between the two shells.

[0041] With an interface layer that easily displaces in response to evenslight tangential forces, for example, it may be desirable to restrictsuch displacement prior to an impact event. Accordingly, FIG. 11A showsan interface layer construction with a primary interface medium 114disposed between an outer helmet layer 110 and an inner helmet layer112. Intermittent connecting members 116 pass through primary interfacemedium 114 to join the inner and outer helmet layers. The connectingmembers 116 are depicted as integral to inner helmet layer 112, but can,in the alternative: be integral to outer helmet layer 110; containsections integral to both the inner and outer helmet layers that areengaged/connected during assembly; or be completely separate from theinner and outer helmet layers until connected during assembly.

[0042] Preferably, connecting members 116 substantially preventdisplacement of the outer helmet layer with respect to the inner helmetlayer under normal usage and handling, thereby imparting a unitary feelto the helmet. For instance, FIG. 11B illustrates the response of theinterface layer 114 with connecting members 116 when a tangential forceF_(tang) less than a design shear force F_(S) is applied to outer helmetlayer 110. A small displacement d is observed as connecting members 116resist the propensity of interface layer 114 to displace.

[0043] Connecting members 116 are designed to shear or otherwisedisconnect, however, when a tangential force F_(tang) exceeds the designshear force F_(S). The design shear force F_(S) is preferably set lowenough that connecting members 116 fail at tangential impact forcesindicative of a crash—and lower than a force that would cause apotentially injurious head acceleration. As shown in FIG. 11C, onceconnecting members 116 fail, interface layer 114 can allow a relativelylarge displacement D.

[0044] In some embodiments, it may not be necessary that members 116actually connect the inner and outer helmet layers. For instance,members 116 can substantially impede large displacements of ahyper-elastic layer by virtue of protruding through a portion of thelayer's thickness and extending along the layer perpendicular to thedirection in which displacement is to be constrained.

[0045] Another way to view an interface layer with connecting and/ordisplacement-impeding members is as an interface layer that respondsnon-linearly to tangential forces. FIG. 12 contains a graph 120 showingdisplacement of an outer layer 122 with respect to an inner layer 124 asa function of an applied tangential force F_(tang). Aconnecting/displacement-impeding member 128 within an interface layer126 initially resists tangential forces less than F_(PEAK) (shown forillustrative purposes at about 0.6 KN). Forces less than F_(PEAK) causea relatively small and temporary displacement, with the helmetelastically restoring itself to zero displacement once the force isremoved. Once F_(PEAK) is exceeded, however, member 128 fails, causingan inelastic change in the response characteristic of interface layer126, which can then move through relatively large displacements inresponse to tangential forces much smaller than F_(PEAK).

[0046] Connecting members can take a variety of forms. For instance,FIG. 13 illustrates a cross-section of a vented helmet 130. Helmet 130contains connecting members 139 molded into an inner helmet layer 132 atthe periphery of air vents and along the outer periphery of the innerhelmet layer 132 where it joins the outer periphery of an outer helmetlayer 135. Accordingly, the outwardly facing surface of inner helmetlayer 132 contains formed depressions, between connecting members 139,into which an interface medium 138 can be inserted. Outer helmet layer135 is subsequently positioned as shown and adhered to both interfacemedium 138 and connecting members 139. Thus the cross-sectioned innerhelmet support pillar 131 adheres to cross-sectioned outer helmetsupport pillar 134, encapsulating a portion of interface medium 138 andforming one surface of an air vent and a lower helmet exterior surface.Cross-sectioned inner helmet support pillar 133 adheres tocross-sectioned outer helmet support pillar 136, encapsulating anotherportion of interface medium 138 and forming two air vent surfaces.Similar construction can be used in the rest of the helmet. Thus helmet130 contains an encapsulated interface medium 138 and has the outwardappearance of a prior art helmet. Under a large impact force, however,connecting members 139 readily fail, allowing the outer helmet assemblyto rotate about the inner helmet assembly due to the previouslydescribed properties of interface medium 138.

[0047]FIG. 14 shows the results of a guided free-fall drop testcomparing a standard bicycle helmet with the same helmet type equippedwith a low-friction interface layer. One standard helmet was modified byattaching a distensible silicone-filled envelope to the outside of theouter helmet layer. A segment of an outer helmet layer from an identicalhelmet was then attached to the outside of the silicone-filled envelope,forming a low-friction interface (LFI) helmet.

[0048] Drop tests on the standard and modified helmets were thenperformed by attaching the helmets to a headform with a hinged “neck”joint and dropping each helmet onto a concrete anvil. The concrete anvilhad a top surface angled at 30 degrees to horizontal to simulate anoblique impact that might occur in a bicycle crash where a rider islaunched onto pavement.

[0049] Peak linear acceleration, peak angular acceleration, and neckmoment were measured for the standard and LFI helmets. Upon impact, theinner assembly of the LFI helmet translated parallel to the surface ofthe anvil, inducing a backward rotation of the head around the neckjoint. The standard helmet did not slide on the anvil surface, andtherefore induced a head flexion moment. The head flexion moment furtherform-locked the standard helmet to the anvil, prevented sliding of thestandard helmet. Accordingly, compared to the standard helmet, the LFIhelmet exhibited an 87% smaller peak linear acceleration, a 68% lowerpeak angular acceleration, and a 74% decrease in neck moments.

[0050] Helmet materials are widely selectable, depending on design. Byway of example, energy-absorbing layers can be constructed ofpolystyrene foam, expanded polystyrene foam, hexagonal honeycombstructures, and the like. Some outer shell materials are:titanium/titanium alloys; epoxies; fiberglass-epoxy composites;carbon-fiber-epoxy composites; polyethylene; polycarbonate; andfluoropolymers. Some potential interface layer materials are:silicon-based gels; hyper-elastic materials (e.g., rubber based onlatex, silicon, or polyurethane); and sliding interface layer pairs ofpolyethylene, fluoropolymers, or polycarbonate. Those skilled in the artwill recognize from the preceding disclosure the large number ofpotential combinations of these, as well as other materials notexplicitly listed, that can be combined in an embodiment of the presentinvention.

[0051] One of ordinary skill in the art will recognize that the conceptstaught herein can be tailored to a particular application in many otheradvantageous ways, and that the embodiments presented are merelyexemplary. Some preferred embodiments utilize an interface layer with aspherical curvature, thus allowing rotational displacement of an outerhelmet assembly in a plurality of axes of rotation. Other arrangementsare possible, however. For instance, the helmet layers could containfeatures, such as longitudinal channels or ridges, that constraindisplacement to fore-and-aft rotation. Or, particularly as the arclength of the interface decreases, it could depart significantly from aspherical curvature while still allowing considerable displacement. Adisplaceable outer helmet section that primarily protects the forwardhelmet quadrants could even employ a canted planar interface. Althoughthe innermost layer in the described embodiments was an energy-absorbinglayer, that layer can alternately be a hard layer, with the helmetretention system providing head cushioning.

[0052] Although the specification may refer to “an”, “one”, “another”,or “some” embodiment(s) in several locations, this does not necessarilymean that each such reference is to the same embodiment(s), or that thefeature only applies to a single embodiment.

What is claimed is:
 1. A helmet comprising: an inner helmet layer; anouter helmet layer; and an interface layer disposed between the innerand outer helmet layers, the interface layer allowing displacement ofthe outer helmet layer with respect to the inner helmet layer inresponse to a tangential impact force applied to the outer helmet layer.2. The helmet of claim 1, wherein the outer helmet layer comprises ahard shell, and wherein the inner helmet layer comprises anenergy-absorbing layer.
 3. The helmet of claim 1, wherein the outerhelmet layer comprises a first energy-absorbing layer and a microshellat least partially overlying the first energy-absorbing layer, andwherein the inner helmet layer comprises a second energy-absorbinglayer.
 4. The helmet of claim 1, further comprising a helmet retentionsystem to secure the inner helmet layer to a wearer's head.
 5. Thehelmet of claim 1, the helmet comprising features to constrain theallowable displacement to rotational displacement along a single axis ofrotation.
 6. The helmet of claim 1, wherein the interface layer has aspherical curvature, such that the allowable displacement comprisesrotational displacement in a plurality of axes of rotation.
 7. Thehelmet of claim 6, wherein the interface layer has a substantiallyuniform thickness.
 8. The helmet of claim 1, wherein the interface layersubstantially fills a gap between the inner and outer helmet layers. 9.The helmet of claim 1, further comprising a plurality of air ventspassing through the inner and outer helmet layers to allow air movementbetween the exterior of the helmet and the interior of the helmet. 10.The helmet of claim 9, wherein a connecting member joins the inner andouter helmet layers around the periphery of one or more of the airvents.
 11. The helmet of claim 10, wherein the connecting membersubstantially prevents displacement of the outer helmet layer withrespect to the inner helmet layer, and fails when a tangential impactforce indicative of a crash is applied to the outer helmet layer. 12.The helmet of claim 1, wherein at least one connecting member joins theinner and outer helmet layers, substantially prevents displacement ofthe outer helmet layer with respect to the inner helmet layer, and failswhen a tangential impact force indicative of a crash is applied to theouter helmet layer.
 13. The helmet of claim 12, wherein the connectingmember is positioned around the periphery of the helmet.
 14. The helmetof claim 1, wherein the interface layer responds non-linearly totangential forces, such that tangential forces less than a thresholdforce result in relatively small elastic displacements, and a tangentialforce larger than the threshold force causes an inelastic change in theinterface layer after which tangential forces smaller than the thresholdforce cause relatively large displacements.
 15. The helmet of claim 1,wherein the interface layer comprises an envelope attached to the innerand outer helmet layers, the envelope containing a viscous medium. 16.The helmet of claim 1, wherein the interface layer comprises ahyper-elastic structure.
 17. The helmet of claim 16, wherein thehyper-elastic structure comprises a formable gel.
 18. The helmet ofclaim 16, wherein the hyper-elastic structure comprises anelastomer-based lamellar structure.
 19. The helmet of claim 1, whereinthe outer helmet layer comprises a first solid shell, and the interfacelayer comprises a second solid shell attached to the inner helmet layerand in contact with the first solid shell, such that alloweddisplacement occurs between the first and second solid shells.
 20. Thehelmet of claim 19, further comprising a lubricant disposed between thefirst and second solid shells.
 21. The helmet of claim 1, wherein theinterface layer is adapted to at least partially dampen impact energyapplied normal to the surface of the outer helmet layer.
 22. The helmetof claim 1, wherein the outer helmet layer has a front lower surfacecapable of extending, during a forward displacement, to provideadditional facial protection to a wearer.
 23. The helmet of claim 1,wherein at least one displacement-impeding member protrudes into theinterface layer from the inner or outer helmet layer, the at least onedisplacement-impeding member substantially impedes displacement of theouter helmet layer with respect to the inner helmet layer, and failswhen a tangential impact force indicative of a crash is applied to theouter helmet layer
 24. A method of head protection, comprising: joiningan inner helmet layer, an outer helmet layer, and an interposedinterface layer that allows relative displacement between the inner andouter helmet layers; and in response to a tangential impact forceapplied to the outer helmet layer, displacing the outer helmet layerwith respect to the inner helmet layer.
 25. The method of claim 24,wherein displacing the outer helmet layer with respect to the innerhelmet layer comprises responding to a tangential force less than athreshold force with a relatively small elastic displacement, andresponding to a tangential force larger than the threshold force byinelastically changing the interface layer, after which tangentialforces smaller than the threshold force cause relatively largedisplacements.
 26. The method of claim 25, wherein the interface layercomprises at least one connecting member connecting the outer and innerhelmet layers, and wherein inelastically changing the interface layercomprises severing the connection formed by the connecting member. 27.The method of claim 24, wherein the interface layer has a substantiallyspherical curvature, and wherein displacing the outer helmet layer withrespect to the inner helmet layer comprises rotationally displacing theouter helmet layer with respect to the inner helmet layer.
 28. Themethod of claim 24, wherein the inner helmet layer comprises at least anenergy-absorbing sublayer, the method further comprising absorbingnormal impact forces applied to the outer helmet layer over an extendedarea of the inner helmet layer during the displacement of the outerlayer.
 29. A helmet comprising: an inner helmet assembly having a firstenergy-absorbing layer and a helmet retention system to secure the firstenergy-absorbing layer to a wearer's head; an outer helmet assemblyhaving a second energy-absorbing layer; and an interface layer disposedin a region of substantially spherical curvature between the inner andouter helmet assemblies, the interface layer allowing rotationaldisplacement of the outer helmet assembly with respect to the innerhelmet assembly in response to a tangential impact force applied to theouter helmet assembly.
 30. The helmet of claim 29, further comprising atleast one connecting element to attach the inner and outer helmetassemblies and substantially impede rotational displacement between thetwo assemblies until the helmet is subjected to a significant impactforce.
 31. The helmet of claim 29, further comprising at least onedisplacement-impeding element protruding from either the inner or outerhelmet assembly into the interface layer and substantially impedingrotational displacement between the two assemblies until the helmet issubjected to a significant impact force.
 32. A helmet comprising: aninner helmet assembly having a first energy-absorbing layer and a helmetretention system to secure the first energy-absorbing layer to awearer's head; an outer helmet assembly having a rigid layer; and aninterface layer disposed in a region of substantially sphericalcurvature between the inner and outer helmet assemblies, the interfacelayer allowing rotational displacement of the outer helmet assembly withrespect to the inner helmet assembly in response to a tangential impactforce applied to the outer helmet assembly.
 33. The helmet of claim 32,further comprising at least one connecting element to attach the innerand outer helmet assemblies and substantially impede rotationaldisplacement between the two assemblies until the helmet is subjected toa significant impact force.
 34. The helmet of claim 32, furthercomprising at least one displacement-impeding element protruding fromeither the inner or outer helmet assembly into the interface layer andsubstantially impeding rotational displacement between the twoassemblies until the helmet is subjected to a significant impact force.